Bimodal silica gel, its preparation and use as a catalyst support

ABSTRACT

A bimodal silica gel particle, its method of preparation and its use as a catalyst support in the polymerization of olefins is disclosed. The bimodal silica gel particle exhibits a bimodal pore radius distribution, wherein the silica gel particle has a first average pore radius and a second average pore radius differing by at least about 20 Å. The bimodal silica gel particle is useful as a support for a transition metal-based catalyst to provide a polyolefin having a broad or bimodal molecular weight distribution.

FIELD OF THE INVENTION

The present invention is directed to a silica gel particle having abimodal pore radius distribution, and its method of manufacture; to atransition metal based catalyst including the bimodal silica gelparticle as a support, and its method of manufacture; and to a method ofmaking polyolefins having broad or bimodal molecular weightdistributions by utilizing the catalyst including the bimodal silica gelparticle.

In particular, the present invention is directed to a method ofpreparing a silica gel particle having a bimodal pore radiusdistribution, wherein an aqueous solution of a silicate is added to anaqueous acid solution until a silica hydrogel is precipitated; aging theprecipitated silica hydrogel in the resulting solution to provide asilica hydrogel having a first average pore radius; adding an additionalamount of the silicate solution to the silica hydrogel and the resultingsolution to raise the pH of the solution to at least about 9; adding anacid to the hydrogel and the solution having a pH of at least about 9 toprecipitate additional silicate as a hydrogel having a second averagepore radius on the hydrogel having the first average pore radius; andheat-aging the hydrogel to fix the respective first and second averagepore radii of the hydrogel.

The resulting hydrogel particle is converted into a silica xerogelhaving a first and a second average pore radii, and the bimodal silicaxerogel is useful as a support for a transition metal based catalyst,such as a chromium-based catalyst. The transition metal based catalystis used in the polymerization of an olefin to produce polyolefins havingbroad or bimodal molecular weight distributions.

BACKGROUND OF THE INVENTION

Silica gels have numerous industrial applications, including use assorbents and as catalyst supports, including supports for olefinpolymerization catalysts. Specifically, these olefin polymerizationcatalysts include a catalytic transition metal, such as chromium,deposited on a silica xerogel support that may be activated byhigh-temperature oxidation. Olefins are polymerized in the presence ofsuch catalysts to produce various polyolefins having different molecularweight distributions and melt indices, depending upon the particulartemperature, pressure, solvent, catalyst and other polymerizationconditions employed.

The production of low molecular weight, high melt index polyolefins isof particular interest because such polyolefins are widely used in filmsand sheets, in extrusion coating, in injection and rotational molding,and in similar industrial applications. In many applications, such as inextrusion or molding applications, a polyolefin, such as polyethylene,having a broad or multimodal (e.g. bimodal) molecular weightdistribution exhibits excellent processing characteristics, such as afaster throughput rate with lower energy requirements.

In many applications, polyolefin toughness, strength, and environmentalstress cracking resistance (ESCR) are important properties. Theseproperties are enhanced when the polyolefin has a high molecular weight.However, as the molecular weight of the polyolefin increases, theability to process the polyolefin resin usually decreases. Therefore, byproviding a polyolefin having a broad or bimodal molecular weightdistribution, the important physical properties exhibited by highmolecular weight resins are retained, and advantageous processingproperties, particularly extrudability, of the polyolefin resin areimproved.

When a polyolefin has a bimodal molecular weight distribution, amolecular weight distribution plot (by size exclusion chromatography) ofconcentration of species of specific molecular weight vs. log molecularweight has at least two maxima. The two maxima are characteristic of abimodal molecular weight distribution, and the maxima need not beequivalent in magnitude or widely separated for the resin to exhibit theproperties of a bimodal polyolefin.

Three major techniques have been proposed or used to produce polyolefinresins having a broad or bimodal molecular weight distribution. Onetechnique is post reactor or melt blending of two or more polyolefinshaving different molecular weights. This technique has the disadvantagesof requiring complete homogenization of at least two polyolefin resinsand an attendant high cost. A second technique utilizes multistagereactors, but this technique has a low efficiency and, again, anattendant high cost. The third, and most desirable, technique is thedirect production of a broad or bimodal polyolefin from a singlecatalyst, or catalyst mixture, in a single reactor. Such a process wouldprovide a polyolefin having a broad or bimodal molecular weightdistribution in situ, with the polyolefin components having a differentmolecular weight being intimately mixed on the subparticle level.

The production of a broad or bimodal polyethylene resin by a singlecatalyst, or by a catalyst mixture, in a single reactor has beendisclosed. For example, U.S. Pat. No. 4,025,707 discloses thepreparation of ethylene homopolymers and copolymers of broadenedmolecular weight distribution by utilizing a mixed catalyst comprisingseveral portions of the same or different chromium components, and metalpromoted variations thereof, wherein each portion is activated at adifferent temperature. U.S. Pat. No. 4,560,733 discloses combiningmagnesium and titanium-containing catalyst components. The catalyst isprepared by milling a blend of at least two silica-containing componentshaving different melt index potentials. U.S. Pat. No. 4,918,038discloses a mixed catalyst system, based on vanadium, to control themolecular weight distribution of the polyolefin.

Although such techniques have improved the processing characteristics ofthe polyolefin, the processing advantage has been largely offset by acorresponding decrease in one or more essential physical properties ofthe polyolefin. For example, the polyolefin resins obtained inaccordance with U.S. Pat. No. 4,025,707 have good die swellcharacteristics, acceptable environmental stress cracking resistance(ESCR) and flow properties, but polymer densities are too low to providethe necessary stiffness for blown bottles. In addition, polyolefinproduced in accordance with the catalysts of U.S. Pat. No. 4,560,733have sufficiently high densities (0.960 and greater), but typically aredeficient in their resistance to environmental stress cracking.

A catalyst system that utilized a blended support, wherein differentgrades of a particular type of support, such as silica or alumina,having a different average pore radius are blended, have failed toprovide a polyolefin having a broad or bimodal molecular weightdistribution. Theoretically, a catalyst prepared on such a blendedsupport should provide a broad or bimodal molecular weight polyolefinbecause the differing average pore radii of the different support gradesprovide polyolefins of a different molecular weight. However, becausesuch a blended support failed to provide a bimodal polyolefin,investigators have continued to seek a catalyst support that exhibits abimodal pore radius distribution and that, when used as a catalystsupport, provides a polyolefin with a broad or bimodal molecular weightdistribution.

Several patents disclose that preparation of a bimodal alumina particle,or a bimodal silica-alumina particle. For example, Murata in U.S. Pat.No. 3,949,030 discloses a cellular fused silica having a bimodal closedcell structure produced from a mixture of silica and boron oxynitride.Heating the silica. boron oxynitride mixture to the melting point of thesilica decomposes the boron oxynitride, thereby releasing a gas thatcreates cells in the silica and providing a silica matrix that exhibitsa bimodal pore diameter. Several other patents, for example, Leach U.S.Pat. No. 3,898,322; Kim et al. U.S. Patent Nos. 4,257,922 and 4,294,685;and Bouge et al. U.S. Patent No. 4,315,839, each disclose an aluminathat exhibits bimodal pore diameter distributions. Such bimodal aluminasare useful as components of hydroprocessing catalysts.

R. Snel, in a series of four publications in Applied Catalysts, 11 p.271-280 (1984); 12 p. 189-200 (1984); 12 p. 347-357 (1984) and 33 p.281-294 (1987), disclosed silica-alumina gels having a bimodal porestructure. Snel also disclosed the use of a nitrogen base as a poreregulating agent. The bimodal silica gel particle of the invention, onthe other hand, is essentially free of alumina (e.g. includes less thanabout 5%, and preferably less than about 2%, alumina), does not includea pore regulating agent, and is prepared by a method that issubstantially different from the method employed by R. Snel.

As previously stated, it would be highly desirable to produce a bimodalpolyolefin directly in a single reactor, i.e., without the need to blendpolyolefins having different molecular weight distributions in order toobtain the advantages of a bimodal polyolefin. It is even more highlydesirable to provide a high activity polymerization catalyst thatproduces high quality polyolefins having a broad or bimodal molecularweight distribution. To achieve this goal, it would be desirable toprovide a silica catalyst support wherein each silica xerogel particleexhibits a bimodal pore radius distribution. The present inventionprovides a silica xerogel particle that exhibits a bimodal pore radiusdistribution and provides catalysts that yield polyolefins having abroad or bimodal molecular weight distribution.

Therefore, the invention is directed to a silica gel particle thatexhibits a bimodal pore radius distribution, and that is used as asupport for a catalyst that yields polyolefins having broad or bimodalmolecular weight distributions. The silica xerogel support is preparedfrom a silica hydrogel prepared by the method of the present invention.The silica hydrogel is prepared by first and second precipitations ofthe silica hydrogel from an aqueous solution, at two distinct pH values,to provide a silica hydrogel particle that exhibits a bimodal poreradius distribution. The difference in pore radius between a firstaverage pore radius and a second average pore radius in the same silicaxerogel particle is at least about 20 Å (angstroms), therebydemonstrating the bimodal properties of the silica hydrogel particle.Converting the silica hydrogel particle to a silica xerogel particledoes not destroy the bimodal pore radius distribution of the silica gelparticle.

In general, the bimodal silica gel of the invention is prepared byadding an aqueous silicate solution to an aqueous acid solution toprecipitate a silica hydrogel in an acidic medium. After an aging step,an additional amount of the silicate solution is added to the aqueousmixture including the silica hydrogel until the pH of the aqueousmixture is raised to at least about 9. Then, an acid solution is addedto the mixture having a pH of at least about 9 to precipitate additionalsilicate on the silica hydrogel in an alkaline medium. The average poreradius of the silica hydrogel precipitated in the first precipitation isdifferent from the average pore radius of the silica hydrogelprecipitated in the second precipitation. Accordingly, a silica hydrogelparticle that has a bimodal pore radius distribution within a singleparticle, and that is essentially free of alumina, is provided. Uponfurther processing, the bimodal silica hydrogel is converted into abimodal silica xerogel. The resulting bimodal silica xerogel then isuseful as a support for a polymerization catalyst. The polymerizationcatalyst, such as a chromium-containing catalyst, provides a polyolefinthat demonstrates a broad or bimodal molecular weight distribution.

The precipitation of a silica hydrogel by adding an aqueous silicatesolution to an aqueous acid solution is disclosed in Stoewener U.S. Pat.No. 1,738,315. Stoewener discloses adding the silicate solution to asufficient amount of acid solution to neutralize the silicate. Stoewenerdoes not teach or suggest a second precipitation of the silicate, anddoes not teach or suggest controlling the pH in the precipitation stepsto provide a bimodal silica hydrogel. Nozemack et al., in U.S. Pat. No.4,780,446, disclose an alumina-silica cogel that includes from about91.5 to about 94.5 percent by weight alumina and that has a wide poresize distribution. The cogel of Nozemack et al. is prepared by adding analuminum sulfate solution, a sodium aluminate solution and sodiumsilicate solution to a water heel, and maintaining the pH of the mixturein the range of 7.6 to 8.4. After the addition of the ingredients to thewater heel, additional sodium aluminate solution is added to the mixtureto increase the pH of the mixture to 9.6 to 10.3. Nozemack et al. do notteach or suggest a second precipitation to provide a bimodalsilica-alumina hydrogel, and Nozemack et al. do not teach or suggest abimodal silica hydrogel particle that is essentially free of alumina,such as a bimodal silica hydrogel particle including less than about 5wt. % alumina.

It is evident that a polyolefin exhibiting a broad or bimodal molecularweight distribution is desirable, or necessary, in several industrialapplications. Attempts to provide such a polyolefin in a single reactorusing a single catalyst, and thereby obviate the blending of polymershaving a different molecular weight distribution, or of blendingcatalysts, has not been entirely successful. Therefore, the invention isdirected to providing polyolefins having broad or bimodal molecularweight distributions from a single catalyst in a single reactor, whereinthe catalyst support is a silica xerogel particle that is characterizedby a bimodal pore radius distribution and is essentially free ofalumina.

SUMMARY OF THE INVENTION

The invention is directed to a silica xerogel particle that exhibits abimodal pore radius distribution. The silica xerogel particle is usefulas a support for a polymerization catalyst, such as achromium-containing catalyst, that produces polyolefins (e.g.polyethylene) having broad or bimodal molecular weight distributions. Inparticular, the silica xerogel particle is prepared from a silicahydrogel particle. The silica hydrogel particle is prepared by a methodwherein two distinct precipitations of a silicate, under conditions ofcontrolled pH, provide a silica hydrogel particle having a first averagepore radius and a second average pore radius that differ by at least 20Å.

In accordance with an important feature of the invention, a silicahydrogel particle having a bimodal pore radius distribution is preparedby the steps of:

(a) forming a silica hydrogel by neutralizing an aqueous solution of asilicate, wherein the silicate includes a cation selected from the groupconsisting of alkali metals, ammonium, and combinations thereof, byadding the silicate solution to a first aqueous acid solution to raisethe pH of the first aqueous acid solution until the silica hydrogel isprecipitated;

(b) aging the silica hydrogel of step (a) in the resulting solution ofstep (a) for a time sufficient to provide an aged silica hydrogel havinga first average pore radius;

(c) adding silicate solution as defined in step (a) to the aged hydrogeland solution of step (b) to raise the pH thereof to at least about 9;

(d) neutralizing the resulting hydrogel and solution of step (c) byadding a second aqueous acid solution thereto to sufficiently lower thepH thereof to further precipitate the silicate as a hydrogel having asecond average pore radius on the hydrogel of step (b); and,

(e) heat-aging the resulting hydrogel and solution of step (d) for asufficient time and at a sufficiently high temperature to fix therespective first and second average pore radii of the hydrogel of step(d), wherein the first average pore radius is different from the secondaverage pore radius.

In accordance with another important feature of the invention, thesilicate solution is added to the first aqueous acid solution in step(a) of the foregoing method over a time period of from about 5 minutesto about 20 minutes to raise the pH of the first acid solution to about4.5 to about 7. Furthermore, after raising the pH to at least about 9 instep (c), a sufficient amount of the second aqueous acid solution isadded in step (d) to lower the pH of the mixture of step (c) to about 5to about 7. This method provides a silica hydrogel particle having abimodal pore radius distribution, wherein the second average poreradius, resulting from the precipitation in an alkaline medium in step(d), is different from the first average pore radius, resulting from theprecipitation in an acidic medium in step (a), with the differencebetween the second average pore radius and the first average pore radiusbeing at least about 20 Å.

In accordance with another feature of the invention, a silica xerogelexhibiting a bimodal pore radius distribution is prepared by a methodwherein two distinct precipitations of a silicate, under conditions ofcontrolled pH, first provide a silica hydrogel particle having a bimodalpore radius distribution. The silica hydrogel is suitably aged, thenwashed with water. Next, water is removed from the washed hydrogel toprovide a silica xerogel having a bimodal pore radius distribution. Thebimodal silica xerogel then can be calcined, and used as a support foran olefin polymerization catalyst. A further feature of the invention isto provide a polyolefin having a broad or bimodal molecular weightdistribution from a chromium based polymerization catalyst including abimodal silica xerogel as a support.

The method of preparing a silica hydrogel and a silica xerogel having abimodal pore radius distribution, the method of preparing an olefinpolymerization catalyst including the bimodal silica xerogel as acatalyst support, and the polymerization of an olefin to produce apolyolefin having a broad or bimodal molecular weight distribution, andother aspects and advantages of the present invention, will becomeapparent from the following detailed description of the preferredembodiments taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of pH vs. time illustrating two distinctprecipitations and the aging periods utilized in a method of preparing asilica hydrogel particle having a bimodal pore radius distribution;

FIGS. 2 through 5 are plots of incremental pore volume (Des(Dv(r))) v.radius, in angstroms, illustrating the pore radius distribution of aphysical blend of two silica xerogels having different pore radiusdistributions; and

FIGS. 6 through 26 are plots of incremental pore volume (Des(Dv(r))) v.radius, in angstroms, illustrating the pore radius distribution ofsilica xerogel particles made either in accordance with the method ofthe invention or in comparative examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a bimodal silica xerogel particle that can beused as a catalyst support for a polymerization catalyst that yieldspolyolefins having broad or bimodal molecular weight distributions. Thebimodal silica xerogel is a particle having two average pore radii thatdiffer by at least about 20 Å. Accordingly, polyolefins having broad orbimodal molecular weight distributions can be produced without the needto blend polyolefins having different average molecular weights, andwithout the need to utilize two catalysts or two reactions that providedifferent molecular weight polyolefins.

In general, the bimodal silica xerogel particle is produced from abimodal silica hydrogel particle. The bimodal silica hydrogel particleis produced by a method wherein an aqueous solution of a silicate (e.g.sodium silicate) first is added to a dilute acid solution (e.g. an about2% to about 12% by weight sulfuric acid solution) until the pH of theacid solution is raised sufficiently (e.g. to about 6) to precipitate asilica hydrogel. The resulting mixture, including the silica hydrogel,then is allowed to age, (e.g. as for about one hour) to provide a silicahydrogel having a first average pore radius. Next, additional,unneutralized silicate solution is added to the mixture until the pH ofthe mixture is at least about 9; followed by the addition of a seconddilute acid solution to the mixture having a pH of at least about 9 toreduce the pH of the mixture to, for example, about 5 to about 7. As thepH is reduced, additional silicate precipitates on theoriginally-precipitated silica hydrogel. The silica hydrogelprecipitated in the second precipitation step has a larger average poreradius than the silica hydrogel precipitated in the first precipitationstep, and accordingly a bimodal pore radius silica hydrogel is provided.

The bimodal silica hydrogel then is heat-aged at about 90° C. and pHabout 6 to about 7. The bimodal silica hydrogel is converted to abimodal silica xerogel first by water washing the bimodal silicahydrogel to reduce the sodium ion concentration of the silica hydrogelslurry to less than about 10 ppm (parts per million), filtering, washing(e.g. with acetone) and drying. The resulting bimodal silica gel productthen is heated at about 100° F. for about two hours in a vacuum oven toprovide a silica xerogel that has first and second average pore radii.

The bimodal silica xerogel is useful as a catalyst support for achromium-based or other catalyst, wherein the catalyst providespolyolefins having broad or bimodal molecular weight distributions. Inaddition, the bimodal silica xerogel is useful in other standardapplications for silica gels, such as, but not limited to, a catalystper se, a support for a catalyst, selective adsorption, and otherstandard applications known to those skilled in the art.

Accordingly, it has been found that the inventive method of preparing asilica hydrogel provides a silica hydrogel particle, and subsequently asilica xerogel particle, having a bimodal pore radius distribution,wherein the difference between the first average pore radius and thesecond average pore radius is at least about 20 Å. The bimodal silicaxerogel particle, when used as a support for a polymerization catalyst,provides polyolefins having broad or bimodal molecular weightdistributions.

This result is new and unexpected in the art in view of previousattempts to provide a broad or bimodal molecular weight distributionpolyolefin from a chromium-based catalyst that utilized a physical blendof two silica xerogel particles having a different average pore radiusas the catalyst support. Such physically-blended catalysts provided abimodal or broad molecular weight distribution polyolefin that exhibitedgood processing properties. However, the present bimodal silica xerogelprovides a bimodal or broad molecular weight distribution polyolefinexhibiting much improved processing properties over polyolefins producedfrom a blended catalyst. These improved processing properties areachieved because the bimodal silica xerogel provides a more intimatemixing of polymer chains at the molecular level. Accordingly, a polymersynthesized from a catalyst including a bimodal silica gel particle ofthe present invention exhibits an improved environmental stress crackingresistance.

It has been found that particular parameters in the method of preparingthe silica hydrogel particle of the present invention are important inproviding a silica hydrogel particle exhibiting bimodal pore radiuscharacteristics. These important parameters include the use of twodistinct silica hydrogel precipitations, control of the pH, and agingperiods after each precipitation. Therefore, in summary, silica hydrogelparticles having a bimodal pore radius distribution can be prepared froma silicate by first precipitating the silicate in an acid medium,followed by aging the precipitated silica hydrogel to form the firstaverage pore radius of the silica hydrogel particle; then precipitatingadditional silicate on the silica hydrogel particle in an alkalinemedium to form the second average pore radius of the silica hydrogelparticle. The bimodal silica hydrogel particle can be converted into abimodal silica xerogel particle without destroying the bimodalproperties of the silica hydrogel.

It has been theorized, but is not relied upon herein, that, after the pHis raised to at least about 9 by the addition of additional silicate, aportion of the surface of the first precipitated silica hydrogel ispartially redissolved and thus prepares the surface of the silicahydrogel for further reaction. The second precipitation is performed inthe alkaline medium that partially dissolves the surface of the firstprecipitated hydrogel.

In particular, the preparation of a bimodal silica hydrogel of thepresent invention is illustrated in FIG. 1, showing the pH rangeswherein the first and second silica hydrogel precipitations occur andthe aging periods after each precipitation. From FIG. 1, the first stepin the method of preparing a silica hydrogel having a bimodal pore sizedistribution includes precipitating a silicate from an aqueous solutionby adding a silicate solution to an aqueous acid solution. This firstsilica hydrogel precipitation is identified in the graph of FIG. 1 asregion A.

In this first precipitation, an aqueous solution of a silicate,including from about 10% to about 40% by weight of silica, as SiO₂, isutilized. In general, any water-soluble silicate can be used as long asthe cation of the silicate can be rinsed from the silica hydrogel. Ifthe cation is not sufficiently removed from the silica hydrogel, thesilica hydrogel can undergo a pore collapse during high temperatureactivation. Usually, the silicate solution is an alkaline solution, andincludes a silicate having a cation selected from the group consistingof an alkali metal, ammonium and combinations thereof. Examples of auseful silicate include, but are not limited to, sodium silicate,potassium silicate, lithium silicate, ammonium silicate and combinationsthereof. Preferably, the silicate utilized in the method of theinvention is sodium silicate, potassium silicate or a combinationthereof. To achieve the full advantage of the present invention, thesodium silicate has a sodium oxide (Na₂ O) to silicon dioxide (SiO₂)ratio in the range about 1:3 to about 1:3.75, and especially a ratio ofabout 1:3.22. The potassium silicate has a potassium oxide (K₂ O) toSiO₂ ratio in the range of about 1:1.8 to about 1:3.3. In addition, thesilicate solution is essentially free of alumina (Al₂ O₃), said silicatesolution including less than about 5%, and preferably less than about 2%by weight, based on the weight of SiO₂ in the silicate solution. Toachieve the full advantage of the present invention, the silicatesolution includes less than about 1% by weight Al₂ O₃.

The aqueous solution of the silicate is added to an aqueous acidsolution to raise the pH of the aqueous acid solution until a silicahydrogel is precipitated. In general, the aqueous acid solution is adilute acid solution including from about 2% to about 12% by weightacid. Preferably, the acid solution includes from about 2% to about 6%by weight acid. However, it should be understood that this is apreferred range for the amount of acid in the aqueous acid solutiontaking into account practical volumes and sufficient acid strength.Amounts of acid in the aqueous acid solution below 2% by weight andabove 12% by weight also are useful in the first precipitation reaction.

In addition, the identity of the acid included in the aqueous acidsolution is not critical, as long as the acid has a sufficient strengthto neutralize the alkaline silicate. Therefore, suitable acids include,but are not limited to, sulfuric acid, nitric acid, phosphoric acid,hydrochloric acid and combinations thereof. Preferably, sulfuric acid isused as the acid in the dilute acid solution, as it has the advantage ofgenerating no corrosive fumes, and the sulfate anion is rinsed from thesilica hydrogel easily.

The aqueous silicate solution is added to the acid solution slowly, suchas over a time period of from about 5 minutes to about 20 minutes, andusually over a time period of from about 10 to about 15 minutes. Theneutralization reaction is exothermic, with the reaction mixturetemperature generally rising from ambient temperature to about 30° C. toabout 35° C. In a case wherein the exothermic reaction causes thetemperature to rise over about 40° C., the reaction mixture can becooled externally to maintain the temperature below about 35° C. Theaddition of the silicate solution to the aqueous acid solution raisesthe pH of the acid solution, and the first silica hydrogel precipitationoccurs in an acidic medium when the pH of the reaction mixture is about4.5 or above, and preferably about 6 or above, such as about 6.3 toabout 6.5. Essentially all of the available silicate is precipitated ata pH of about 6 or above. Generally from about 10 % to about 80%, andpreferably from about 20% to about 60%, by weight of the total silicatepresent in the bimodal silica hydrogel is precipitated in this firstprecipitation. In the subsequent second precipitation, conducted in analkaline medium, the remainder of the silicate is precipitated on thesilica hydrogel precipitated in the acidic medium.

After the first silica hydrogel is precipitated at a pH of about 4.5 orabove, and preferably at about 6 or above, the reaction mixture,including the first precipitated hydrogel, is aged for a time period offrom about 15 minutes to about 4 hours, and preferably for about 30minutes to about 3 hours. The aging step is performed at ambienttemperature, or slightly above ambient temperature if the exothermicneutralization reaction increased the temperature of the reactionmixture. This first aging step is identified in FIG. 1 as region B.

The first aging step, i.e. the second step in the method of preparing abimodal silica hydrogel, is important because the aging step allows apartial condensation of the water associated with the silica hydrogel.However, the aging step is terminated before the silica hydrogel iscompletely condensed, otherwise the silica hydrogel particles becomeexcessively hard and dense. In general, the aging step is of sufficientduration to provide a stable hydrogel, and is terminated beforeexcessive settling of the silica hydrogel precipitate from the reactionsolution occurs. The first aging step also effectively fixes the firstpore radius distribution in the range of about 20 Å to about 150 Å, andeffectively fixes the first average pore radius of the bimodal silicagel in the range of from about 40 Å to about 125 Å, and preferably inthe range of about 50 Å to about 100 Å.

After the first aging step, an additional amount of the silicatesolution is added at a uniform rate, and under agitation, to the mixtureof the precipitated hydrogel and solution of pH about 4.5 to about 7 toraise the pH of the mixture to at least about 9, and preferably to atleast about 10. This step of the present method is illustrated in FIG. 1as region C. It has been theorized, but not relied upon herein, that theincrease in pH partially resolubilizes the precipitated hydrogel andruptures a portion of the silicon-oxygen bonds in the silica hydrogeland provides a number of silicon atoms having two hydroxyl groupsubstituents. In effect, the increased pH opens the outer surface of thesilica hydrogel and a pore structure develops on the outer surface.During the addition of the silicate solution, the pH of the mixtureshould not exceed a pH of about 11, or the previously precipitatedsilica hydrogel can be totally resolubilized. If the silicate additioncauses a rise in pH above about 11, additional acid can be added to themixture to reduce the pH to a suitable level.

After an optional aging period to allow the precipitated silica hydrogelto interact with the alkaline medium, such as to partially open the porestructure on the surface of the silica hydrogel, and illustrated in FIG.1 as region D, the second silica hydrogel precipitation is achieved byadding a dilute aqueous acid solution to the mixture of precipitatedsilica hydrogel and silicate solution having a pH of at least about 9.The reduction in pH resulting from the acid addition to the mixture isillustrated in FIG. 1 as region E. The dilute aqueous acid solutiongenerally includes from about 2% to about 12%, and preferably from about4% to about 8%, by weight acid. The acid solution is added at a uniformrate, under agitation, to reduce the pH of the reaction mixture.

The second silica hydrogel precipitation occurs at a pH of between about8 and about 9, and usually between about 8.3 and about 8.7. The secondprecipitation reaction is continued until precipitation is complete,generally by adding the acid solution until a pH of about 6 is attained.The second silicate precipitation occurs on particles of silica hydrogelthat were formed in the first precipitation, and provides a secondaverage pore radius of the silica hydrogel that differs from the firstaverage pore radius of the silica hydrogel of the first precipitation,thereby providing a bimodal silica hydrogel particle. The second poreradius distribution generally is in the range of about 50 Å to about 500Å.

After the second precipitation step, the mixture of the precipitatedhydrogel and solution of about pH 6 optionally can be aged at ambienttemperature prior to heat-aging the silica hydrogel and solution. Thissecond optional aging step is illustrated in FIG. 1 as region F. Thisoptional aging step can continue for a time period up to 24 hours, andhelps condense the silica hydrogel to provide a more dense silica gelparticle.

After the second precipitation step and the second optional aging step,the silica hydrogel and the solution of about pH 6 are heat-aged, undermild agitation, at a temperature of about 70° C. to about 100° C., andpreferably from about 80° C. to about 95° C., for at least 0.5 hour, andpreferably from about 1 to about 2 hours. The heat-aging step serves totemper the bimodal silica hydrogel by giving the silica hydrogelsufficient structural integrity for a subsequent high temperatureactivation, and also serves to fix the first and second average poreradii of the bimodal silica hydrogel.

The above-described method provides a silica hydrogel particle having abimodal pore radius distribution, wherein the first pore radiusdistribution is in the range of about 20 Å to about 150 Å and the secondpore radius distribution is in the range of about 50 Å to about 500 Å,and wherein the average pore radius of the silica hydrogel precipitatedin the second alkaline precipitation generally is greater than theaverage pore radius of the silica hydrogel precipitated in the firstacidic precipitation. However, it is envisioned that a particular set ofprecipitation conditions can provide a bimodal silica gel particlehaving a first average pore radius that is greater than the secondaverage pore radius. In general, the bimodal silica hydrogel particlehas a first average pore radius of about 40 Å to about 125 Å, and asecond average pore radius of about 60 Å to about 450 Å. Usually, thedifference between the second average pore radius and the first averagepore radius is at least about 20 Å.

After heat-aging the silica hydrogel particle to fix the first andsecond average pore radii, the silica hydrogel is converted to a silicaxerogel. First, the solution of pH about 6 is decanted from the bimodalsilica hydrogel. The slurry of bimodal silica hydrogel particles then iswashed with water to remove the cations from the silica hydrogel slurry,for example to remove sodium ions such that less than about 10 ppm(parts per million) sodium ion remains in the bimodal silica hydrogelslurry. Next, water is removed from the silica hydrogel particles, suchas by rinsing with a water-miscible organic liquid, like, for example, alow molecular weight ketone, a low molecular weight alcohol, or acombination thereof, e.g. acetone, methanol, ethanol or isopropylalcohol, to displace water from the hydrogel. Rinsing with the organicliquid is continued until less than 1% by weight water remains in thebimodal silica hydrogel. The resulting bimodal silica gel product thenis dried under reduced pressure, preferably at a low temperature, suchas at from about 80° C. to about 110° C. to provide a bimodal silicaxerogel. The resulting bimodal silica xerogel has a nitrogen pore volumein the range of about 1.5 to about 3 cc/g, a surface area in the rangeof about 200-500 m² /g, and a bimodal pore radius distribution.

As an alternative to rinsing the silica hydrogel particles with anorganic liquid to displace water from the silica hydrogel, other methodsare available to remove water from the silica hydrogel withoutdestroying the pore structure of silica hydrogel. In each method, wateris removed from the washed silica hydrogel in a manner that avoidssubstantial collapse of the pores of the silica hydrogel. For example,water also can be removed by azeotropic distillation with a compoundcapable of forming an azeotrope with water, such as ethyl acetate orbenzene.

Another method entails washing the silica hydrogel with a water-miscibleorganic liquid or mixture of liquids to remove about 90 to about 95% ofthe water, and then treating the partially dehydrated silica gel with aketal of the formula RC(OR)₂ R, wherein R is the same or different andis an alkyl group including one to five carbon atoms, plus a trace ofacid, whereby residual water is completely and irreversibly removed by achemical reaction. In yet another method, water is removed by contactingthe silica hydrogel directly with a sufficient amount of ketal in thepresence of a trace amount of acid as a catalyst. Examples of suitableketals include, but are not limited to, 2,2-dimethoxypropane,2,2-dimethoxybutane, 2,2-diethoxypropane, 2-methoxy-2-ethoxypropane andsimilar ketals. Each of the above-described water removal methods isdisclosed in Dombro et al. U.S. Pat. No. 4,246,137; Dombro U.S. Pat. No.4,279,780; and Dombro et al. U.S. Pat. No. 4,791,089, each incorporatedherein by reference.

The bimodal silica xerogel then has a chromium catalyst depositedthereon, and usually, is subjected to a high temperature activation. Ingeneral, the bimodal silica xerogel incorporates a chromium catalystbecause chromium catalysts provide polyolefins having a broad molecularweight distribution. In contrast, the titanium-type Ziegler catalystsproduce polyolefins having a very narrow molecular weight distribution.

To demonstrate the ability of the above-described method to provide abimodal silica hydrogel and xerogel, several silica gels were prepared,under varying method parameters. It has been shown that a bimodal silicaxerogel has been prepared, wherein the bimodal silica xerogel has a porevolume of over 2 cc/g and a surface area of 200 to 500 m² /g. Thebimodal silica xerogel was coated with a chromium-containing catalyst,such as chromium acetoacetonate or chromium phosphoryl complex, wasactivated and was used in a polymerization reaction to produce a bimodalor broad molecular weight distribution polyethylene.

In summary, from the following Examples and Comparative Examples, it wasfound that a bimodal silica hydrogel was prepared by conducting twodistinct hydrogel precipitations in the same batch, but under differentconditions. The first precipitation occurs in an acidic medium by addingan aqueous sodium silicate solution to a dilute sulfuric acid solution.After a one hour aging period, additional sodium silicate solution isadded to the mixture to bring the pH to at least about 9. Then, a diluteacid solution is added to precipitate the second portion of the hydrogelin an alkaline medium and to neutralize the slurry. The inventive methodhas been performed both in a stirred, low shear vessel and in a highshear reactor. In particular, the following Examples and ComparativeExamples demonstrate that:

high shear or low shear agitation can be used to provide a bimodalsilica hydrogel;

two silicate precipitations in the same batch, under conditions ofcontrolled pH, provide a bimodal silica gel;

the silica hydrogel is aged for at least 15 minutes after the firstprecipitation to provide the first average pore size distribution;

between the first and second precipitations, additional silicate salt isadded until the mixture attains a pH of at least about 9;

acid concentrations between about 2% and about 12% by weight can beused, with a 2% by weight acid concentration being preferred for thefirst precipitation and a 4% by weight acid concentration preferred forthe remainder of the process; and

the bimodal silica hydrogel can be heat-aged essentially immediatelyafter the second precipitation or after an overnite aging period;however, a bimodal silica hydrogel having a higher pore volume isobtained when the bimodal silica hydrogel is heat-aged essentiallyimmediately after the second precipitation.

Each of the bimodal silica xerogels of the following Examples wasprepared by the general method described in Example 1. As will bediscussed more fully hereinafter in individual Examples and ComparativeExamples, various parameters were changed to determine the effect ofthat parameter on the bimodality of the silica xerogel.

EXAMPLE 1 General Method of Preparing the Silica Xerogels of Examplesand Comparative Examples 2 Through 19

A sufficient amount of an aqueous acid solution including from about 2%to about 12% by weight sulfuric acid is added to a reaction vessel. Theacid solution is agitated, and a sodium silicate solution then ismetered into the agitated acid solution. At the onset of silica hydrogelprecipitation, i.e. the first gel point, the addition of sodium silicatesolution, and agitation, are stopped for an approximately one hour agingperiod.

After aging, agitation is resumed, then addition of the silicatesolution to the reaction vessel is resumed. If necessary, a dilute acidsolution also is added to control the maximum pH of the resulting slurrybelow about 11. When the entire amount of the sodium silicate solutionhas been added to the slurry, an aqueous acid solution including fromabout 2% to about 12% by weight sulfuric acid is added to the reactionvessel until the second precipitation point is reached at about pH 8 to9. The slurry in the reaction vessel then is essentially completelyneutralized by adding additional acid solution until the contents of thereaction vessel attain a pH of between about 5 and about 7. After anappropriate hold time specified for that particular Example orComparative Example, the slurry is heat-aged by heating the slurry toabout 90° C. and maintaining the slurry at about 90° C. for about onehour while controlling the pH at about 6.5.

The heat-aged slurry including the silica hydrogel is washed with waterto remove the sodium sulfate byproduct by decanting successive waterwashings until the sodium ion in the water washings is less than about10 ppm. The water slurry then is filtered to remove the excess water.Next, the silica hydrogel cake is washed with acetone to displace thewater in the pores of the bimodal silica. The acetone washings arecontinued until the resulting bimodal silica gel product includes lessthan 1% by weight water, i.e. the acetone wash has a specific gravity of0.793 or less. The acetone slurry then is filtered, and the recoveredbimodal silica gel product is dried in a vacuum oven for two hours at110° C. and 28 in. Hg (inches mercury) vacuum. A sample of the resultingbimodal silica xerogel is analyzed for pore volume, surface area andpore radius distribution after heating the silica xerogel at about 1000°F. for about 4 hours.

The pore radius distribution of the silica xerogel was determined on aporosimeter. To demonstrate that a porosimeter detects a silica xerogelhaving a bimodal pore radius distribution, dry mixtures ofcommercially-available silica xerogels having different average poreradius distributions were prepared, then tested on the porosimeter. Thesilica xerogels were tested in accordance with a standard procedure,ASTM Method D 4641 Pore Size Distribution of Catalysts from NitrogenDesorption Isotherms, that is well known to those skilled in the art.The test results were plotted on graphs of pore radius (in angstroms)versus incremental pore volume (Des(Dv(r))) to provide a visualdepiction of the pore radius distribution of the tested silica gel.

For example, FIG. 2 shows the average pore radius distribution of thecommercial silica gel, POLYPOR® Silica Gel, available from USI Division,Quantum Chemical Corp., New York, N.Y. and having an average pore radiusof about 150 Å. Similarly, FIG. 3 shows the average pore radiusdistribution for the commercial silica gel, MS-967, available fromDavison Chemical Div., W.R. Grace and Co., Baltimore, Md., and having anaverage pore radius of about 50 Å. FIG. 4 shows that a blended mixtureincluding 1 part POLYPOR® Silica Gel and 2 parts MS-967, by weight, whentested on a porosimeter, gave pore radius distribution peaks identicalto the peaks of the individual silica xerogels. FIG. 5 shows similarresults for a mixture including 2 parts POLYPOR® Silica Gel and 1 partMS-967 by weight. FIGS. 4 and 5 are illustrative of a silica xerogelthat exhibits a bimodal pore size distribution. However, it should beunderstood that in FIGS. 4 and 5 the difference in average pore radiusoccurs between individual silica xerogel particles, whereas a bimodalsilica xerogel prepared by a method of the present invention has twodifferent average pore radii in the same silica xerogel particle. Italso should be understood that a blend of silica xerogels, asillustrated in FIGS. 4 and 5, when used as a catalyst support for achromium-based olefin polymerization catalyst, produce a polyolefin thatexhibits a broad or bimodal molecular weight distribution, but thepolymer chains are not commingled at a molecular level. In contrast, abimodal silica xerogel of the present invention did provide a polyolefinhaving a broad or bimodal molecular weight distribution and havingpolymer chains commingled at the molecular level, and thereforeexhibited improved processing properties, such as melt flow.

EXAMPLE 2

A bimodal silica xerogel prepared by the method of the invention, asoutlined in Example 1, is illustrated in FIGS. 6 through 10. Inpreparing this particular bimodal silica xerogel, dilute sodium silicate(25% as SiO₂) was added to a heel of aqueous 4% by weight sulfuric aciduntil a precipitate, or gel, formed at about pH 6. The addition of thesodium silicate solution, and agitation, were stopped, then after a onehour aging period, the remainder of the sodium silicate solution wasadded to the aged slurry with agitation. The pH of the slurry was about10.4. Next, a 4% by weight sulfuric acid solution was metered into theslurry and the pH was reduced from about 10.4 to 6.5. The secondprecipitate, or gel, formed at a pH of about 8.3. The resulting slurrywas heat-aged after a 24 hour hold period at ambient temperature.

In particular, FIG. 6 is a pore radius curve, determined by ASTM MethodD 4641, for the bimodal silica xerogel of Example 2 and shows a bimodalpore radius distribution with the two major average pore radiusdistribution peaks having a substantial separation of about 75 Å. Sievefractions on an 80 mesh, 140 mesh, 230 mesh and through a 230 meshscreen, and illustrated in FIGS. 7 through 10 respectively, show thatthe silica xerogel of Example 2 exhibits a bimodal pore sizedistribution in all particle sizes. Accordingly, the method of thepresent invention provides a bimodal silica xerogel having a firstaverage pore radius of about 75 Å and a second average pore radius ofabout 150 Å.

EXAMPLE 3

A silica xerogel was prepared in an identical manner to the silica gelof Example 2, except a high shear mixer was utilized, and the 24 hourhold period before the onset of heat-aging was eliminated. In thisExample, heat-aging was started essentially immediately after the secondprecipitation was complete. FIG. 11 illustrates the pore radiusdistribution curve for the silica xerogel of Example 3, showing twosharp peaks at 80 Å and 130 Å. Accordingly, the silica xerogel ofExample 3 has a bimodal pore radius distribution. In addition, thesilica xerogel of Example 3 exhibited a pore volume of 2.65 cc/g (cubiccentimeter/gram) and a surface area of 554 m² /g (square meter/gram).

COMPARATIVE EXAMPLES 4 AND 5

Silica xerogels were prepared in an identical manner to the silicaxerogel of Example 2, except the aging step after the first silicahydrogel precipitation was eliminated. Although, the silica xerogels ofExamples 4 and 5 showed two average pore radii, the average pore radiiwere not sufficiently different to provide a bimodal silica gel. Thepore radius distribution curves for the silica xerogels of Examples 4and 5 are illustrated in FIGS. 12 and 13, respectively. The followingTable 1 illustrates the differences in the method of preparing thesilica xerogels of Examples 2, 4 and 5, and the differences in theresulting silica xerogels.

                  TABLE 1                                                         ______________________________________                                                First Hydrogel                                                                            First                                                             Precipitate Hydrogel                                                  Example (% of Total Precipitate                                                                              P.V.                                           No.     Precipitate)                                                                              Age Time   (cc/g)                                                                              Bimodal                                  ______________________________________                                        2       33%         53     min.  2.03  yes                                    4       37%         0            2.45  no                                     5       33%         0            2.43  no                                     ______________________________________                                    

Therefore, it was found that the aging period after the first silicahydrogel precipitation was essential for bimodality. The aging periodallows sufficient time for syneresis, wherein the particle of theprecipitated silica hydrogel condenses and frees water from theprecipitated silica hydrogel. The aging period allows the pores of theprecipitated silica hydrogel to shrink, and thereby a smaller averagepore radius distribution is formed in the silica xerogel.

COMPARATIVE EXAMPLE 6

A silica xerogel was prepared in an identical manner to the silicaxerogel of Example 2, except the second precipitation step was omitted.Therefore, after the first silica hydrogel precipitate was formed andheld one hour, the silica hydrogel was heat-aged, washed, and convertedinto a silica xerogel. The pore radius distribution curve of the silicaxerogel of Example 6 showed that the silica xerogel had a pore radiuspeak at about 80 Å, illustrating that the first silica hydrogelprecipitate provides the smaller average pore radius distribution,whereas the second silica hydrogel precipitate provides the largeraverage pore radius distribution. The pore radius distribution curve forthe silica xerogel of Example 6 is illustrated in FIG. 14.

EXAMPLE 7

A silica xerogel was prepared in an identical manner to the silicaxerogel of Example 2, except that for the first precipitation the sodiumsilicate solution was added to a heel that included 2% by weightsulfuric acid rather than 4% by weight acid and was prepared in a highshear reactor. The resulting silica hydrogel was softer and more easilydispersed, and accordingly this method of precipitation is a preferredembodiment of the present invention. FIG. 15 illustrates the pore radiusdistribution for the silica xerogel of Example 7, showing a bimodalsilica xerogel having widely separated major pore radius peaks at 60Åand 170 Å radii.

EXAMPLES 8-10

Silica xerogels were prepared in an identical manner to the silicaxerogel of Example 2, however the effect of varying the hold time afterthe second silica hydrogel precipitation, and prior to heat-aging thesilica hydrogel, on pore volume was investigated. Each of the silicaxerogels of Examples 8-10 demonstrated a bimodal pore radiusdistribution, with the greatest pore volume and surface area found whenthe heat-aging step was performed without a holding period after thesecond precipitation step. For example, the bimodal silica xerogel ofExample 8, with no holding period after the second silica hydrogelprecipitation, had a pore volume (P.V.) of 2.47 cc/g and a surface area(S.A.) of 360 m² /g; the bimodal silica xerogel of Example 9, with a 1.5hour holding period after the second silica hydrogel precipitation, hada P.V. of 1.91 cc/g and an S.A. of 278 m² /g; and the bimodal silicaxerogel of Example 10, with a 24 hour holding period after the secondsilica hydrogel precipitation, had a P.V. of 1.80 cc/g and an S.A. of266 m² /g. The pore radius distributions of the bimodal silica xerogelsof Examples 8.10 are illustrated in FIGS. 16-18, each showing thebimodal pore radius distribution of the silica xerogel.

EXAMPLE 11

A silica xerogel was prepared in an identical manner to the silicaxerogel of Example 2, except the silica xerogel of Example 11 was agedfor 1.5 hours rather than 24 hours after the second silica hydrogelprecipitation step and prior to heat aging. The resulting silica xerogelof Example 11 had a bimodal pore size distribution with a major smallpore distribution radius peak at 60 Å, and wide pore radius distributionpeaks for the large pore radii of 110 Å to 280 Å. This pore radiusdistribution is similar to the pore radius distribution of the silicaxerogel of Example 2 that had wide pore radius distribution peaks from110 Å to 170 Å. The pore radius distribution of the silica xerogel ofExample 11 is illustrated in FIG. 19.

EXAMPLES 12-14

Silica xerogels were prepared in an identical manner to the silicaxerogel of Example 2, except the pH of the slurry after the firstprecipitation was raised to different values before the acid additionprovided the second precipitation. In Example 12, the silica xerogel wasmade under low shear conditions, with a maximum pH of 10.5 and noholding time before the acid addition effected the second silicahydrogel precipitation. The plot of FIG. 20 shows that the silicaxerogel of Example 12 had a bimodal pore radius distribution. In Example13, the silica xerogel was made under low shear conditions, with amaximum pH of 10.3 and a 24 hour holding time before the acid additioneffected the second precipitation. The bimodality of the silica xerogelof Example 13 is illustrated in the plot of FIG. 21.

The silica xerogel of Example 14 was formed under high shear conditions,with a maximum pH of 10.7 and a 24 hour holding time before the acidaddition effected the second silica hydrogel precipitation. The graph inFIG. 22 shows that the bimodality of the silica xerogel essentially hasbeen destroyed. The silica xerogel of Example 14 did not demonstratebimodality because during the conversion of the silica hydrogel to thesilica xerogel, water was removed from the pores of the silica gelparticles by excessive filtering rather than by acetone displacement.From the following TABLE 2, it is observed that the silica hydrogels ofExamples 3 and 14 were prepared under essentially identical conditions.However, the silica xerogel of Example 3 exhibited bimodality, whereasthe silica xerogel of Example 14 did not exhibit bimodality. The silicaxerogel of Example 14 failed to exhibit bimodality because the silicahydrogel of Example 14 was filtered to dryness when filtering water fromthe hydrogel slurry, thereby removing water from the pores in thehydrogel and causing pore collapse. Accordingly, the pore volume andsurface area of the silica xerogel of Example 14 is substantially lessthan the xerogel of Example 3, and, bimodality also was destroyed.

Therefore, it has been demonstrated that a water removal method thatavoids pore collapse is important in providing a bimodal silica xerogelof the present invention. The water removal methods previously discussedeach gently remove water from the silica hydrogel in order to retainpore integrity, and therefore bimodality. This is accomplished byutilizing one of the above described methods that avoid conditionsduring water removal wherein the surface tension of the water beingremoved becomes sufficiently strong to collapse the pores due tocapillary action thereon.

EXAMPLES 15-16

The xerogels of Examples 15 and 16 were xerocogels including silica andtitanium, and made in accordance with the method outlined in Example 1.The xerocogel of Example 15 was made under conditions of low shear,whereas the xerocogel of Example 16 was made under high shearconditions. The titanium compound, present in a minor amount of thexerocogel, was titanium sulfate, e.g. TiSO₄ ·H₂ SO₄ ·8H₂ O. Thexerocogels of Examples 15 and 16, as illustrated in FIGS. 23 and 24,have a broad pore radius distribution, with bimodal characteristics.

Other suitable titanium-containing compounds useful in the preparationof a silica-titanium xerocogel that exhibits a bimodal pore sizedistribution include an alkali metal titanium oxalate, like potassiumtitanium oxalate and ammonium titanium oxalate, i.e. K₂ TiO(C₂ O₄)₂ ·2H₂O and (NH₄)₂ TiO(C₂ O₄)₂ ·2H₂ O, respectively. Furthermore, in place ofor in addition to the titanium, either zirconium or vanadium, or acombination thereof, can be incorporated into a bimodal xerocogel of thepresent invention. Also envisioned is a xerocogel, including acombination of silica with titanium, vanadium and zirconium, thatdemonstrates a bimodal pore size distribution. A suitablezirconium-containing compound is an alkali metal or an ammoniumzirconium oxalate, like ammonium tetraoxalatozirconate(IV) tetrahydrate.A suitable vanadium-containing compound is a vanadium trihalidecompound, a vanadium pentahalide compound, or a vanadium oxyhalidecompound, like vanadium pentachloride, vanadium trichloride, or vanadiumoxytrichloride.

In every case, the titanium and/or zirconium and/or vanadium-containingcompound is present in a minor amount compared to the silica.Preferably, when a xerocogel or a xerotergel is used to support achromium catalyst, the sum of the zirconium and titanium and vanadiumconcentrations with respect to silica, as SiO₂, is about 5 wt. % orless. The titanium and/or zirconium and/or vanadium-containing compoundsare coprecipitated with the silicate salt. Such coprecipitations resultin an intimate incorporation and distribution of the titanium and/orzirconium and/or vanadium into the bulk of the silica. Zirconium oxalatecompounds, when coprecipitated in a silica hydrogel, provide advantagesas catalyst supports for olefin polymerizations because the zirconiumpreserves the pore structure of the calcined xerocogel and providesresins having relatively high melt indexes (i.e. relatively lowmolecular weights), and does not adversely effect the bimodal poreradius distribution of the xerogel.

To summarize, TABLE 2 lists the properties of the silica xerogelsExamples 2 through 16, including bimodal properties, pore volume andsurface area, and various parameters used in the preparation of theparticular silica xerogel.

                                      TABLE 2                                     __________________________________________________________________________                       First  Second  Hold Time                                                      Precipitation                                                                        Hold Time                                                                             For Heat                                           Bimodal                                                                             Agitation                                                                           Aging Time                                                                           and max. pH                                                                           Aging P.V..sup.1)                                                                       S.A..sup.2)                       Example No.                                                                          Properties                                                                          Shear (in minutes)                                                                         (hours/pH)                                                                            (Hours)                                                                             (cc/g)                                                                            (m.sup.2) /g)                     __________________________________________________________________________    2      Yes   Low   53     0/10.35 24    2.02                                                                              274                               3      Yes   High  53     24/10.7 0     2.65                                                                              554                               4      No    Low   0      0/>10   1     2.45                                                                              284                               5      No    Low   0      0/>10   1.5   2.43                                                                              297                               6      No    Low   60     --      1     2.13                                                                              485                               7      Yes   High  70     .25/>10 24    2.31                                                                              375                               8      Yes   Low   56     0/>10   0     2.47                                                                              360                               9      Yes   Low   66     0/>10   1.5   1.91                                                                              278                               10     Yes   Low   66     0/>10   24    1.80                                                                              266                               11     Yes   Low   61     0/>10   2     1.72                                                                              251                               12     Yes   Low   61     0/10.5  0     1.29                                                                              154                               13     Yes   Low   60     24/10.3 0     1.47                                                                              195                               14     No    High  56     24/10.7 0     1.94                                                                              289                               15     Broad.sup.3)                                                                        Low   60     0/>10   24    1.30                                                                              216                               16     Broad.sup.3)                                                                        High  60     5 minutes/>10                                                                         24    1.29                                                                              235                               18     Yes   Low   0      --      --    2.37                                                                              639                               19     Yes   Low   57     0/>10   1.5   1.93                                                                              264                               __________________________________________________________________________     .sup.1) Pore volume;                                                          .sup.2) Surface area; and                                                     .sup.3) Multiple peaks indicating bimodal characteristics.               

As stated above, a bimodal silica xerogel of the invention can be usedas a support for an olefin polymerization catalyst. Preferably, thecatalyst is a chromium-based catalyst. Such chromium-based catalystsprovide a broad molecular weight distribution polyolefin, and areparticularly useful in the polymerization of a 1-olefin (e.g. ethylene),having no branching nearer the carbon-carbon double bond than the4-position. Furthermore, the chromium-based catalyst can be used in thehomopolymerization of such a 1-olefin or in the copolymerization of twoor more such 1-olefins.

The bimodal silica xerogels of the invention also exhibit otheradvantageous properties desired in a catalyst support. For example, thepresent bimodal silica xerogels generally have a pore volume of about 2cc/g or greater. A silica xerogel support having a pore volume ofgreater than 2 cc/g are referred to as high pore volume silica supports,and are well-known in the art as a suitable catalyst support. Especiallyuseful high pore volume silica supports have a pore volume from 2.1 cc/gto 2.9 cc/g. In addition to exhibiting a bimodal radius distribution anda pore volume of greater than about 2 cc/g, the silica xerogel particlesof the present invention also have a surface area in the range of about200 to about 500 m² /g. Such a surface area is regarded as desirable ina catalyst support material. To achieve the full advantage of thepresent invention, the bimodal silica xerogel of the present inventionis calcined at about 400° C. to about 1000° C. for several hours, e.g.up to about 12 hours, after the active catalyst component, such aschromium(III)acetylacetolate, is deposited on the bimodal silica xerogelsupport. Calcining however is not necessary, and useful catalysts areobtained when the calcining step is omitted.

A chromium-based olefin polymerization catalyst, including a bimodalsilica xerogel particle of the invention as a support, providespolyolefins, and especially polyethylenes, that exhibit a broad orbimodal molecular weight distribution. Such polyolefins are especiallyuseful in blow applications, and now can be produced without the need toblend polyethylene resins of different average molecular weights;without the need to blend two or more catalysts; and without the need toperform two or more polymerization reactions.

As stated above, the bimodal silica xerogel particles of the inventionare especially useful as the support material for a chromium-basedcatalyst. The chromium compounds useful as the active catalyst componentof the polymerization catalyst include any chromium-containing compoundcapable of interacting with the surface hydroxyl groups of the bimodalsilica xerogel particle. For example, chromium-containing compoundscapable of interacting with the surface hydroxyl groups of the bimodalsilica xerogel support include, but are not limited to, chromium nitratechromium trioxide; chromate esters, such as chromium acetate, chromiumacetylacetonate, t-butyl chromate and di-tertiary polyalicyclic chromateesters; silyl chromate esters; and phosphorus-containing chromateesters.

In general, a zerovalent chromium, a chromium(II), a chromium(III) or achromium(VI) compound, or combinations thereof, can be the activecatalyst component interacted with the bimodal silica xerogel particle.Nonlimiting examples of chromium(II) compounds include chromocene andcyclopentadienyl chromium ethoxide; nonlimiting examples ofchromium(III) compounds include chromium(III)acetate,chromium(III)propionate and, preferably, chromium(III)acetylacetonate;nonlimiting examples of chromium(VI) compounds includechromium(VI)trioxide, ammonium dichromate and bis-triphenylsilylchromate. Examples of zerovalent chromium compounds include, but are notlimited to, the diarenechromium compounds, such as dibenzene chromium,ditoluene chromium, di-o-xylene chromium, di-p-xylene chromium, dicumenechromium, dimesitylene chromium, di(n-propylbenzene)chromium,di(1,3,5-triethylenebenzene) chromium, di(1,3-diethyl-4-hexylbenzene)chromium, di(1,3-dipentylbenzene) chromium, di-(1,3,5-trihexylbenzene)chromium, di(hexamethylbenzene) chromium, and the like, and mixtures ofany two or more thereof.

The chromium compounds useful as the active catalyst component of thepolyolefin catalyst generally are liquids or solids that are soluble incommon organic solvents. Preferably, the chromium compound is soluble ina nonpolar organic solvent that is sufficiently volatile to allowremoval of the organic solvent from the catalyst by evaporation. Classesof suitable organic solvents for solubilizing the chromium compoundinclude alkanes, cycloalkanes, aromatic hydrocarbons, halogenatedcompounds, ethers, and like non-polar organic liquids. Exemplary organicsolvents for the chromium compound include, but are not limited to,pentane, n-hexane, decane, cyclohexane, methylcyclohexane, benzene,xylenes, chloroform, methylene chloride, diethylether, and similarsolvents, and mixtures thereof. Polar organic solvents, such asmethanol, ethanol, similar low molecular weight alcohols, pyridine andacetone also can be used. It should be understood however that thechromium compound can be introduced onto the bimodal silical xerogelsupport neat, but it is preferred, for ease and efficiency in handlingthe chromium compound, that the chromium compound is solubilized in anorganic solvent.

In one embodiment, an olefin polymerization catalyst, comprising abimodal silica xerogel particle of the present invention as the supportand a chromium compound as the active catalyst component, is produced byadmixing a bimodal silica hydrogel with a water-soluble chromiumcompound, such as chromium(III)acetate, chromium(III)nitrate, chromiumtrioxide or ammonium dichromate, then spray drying the resultingmixture. Alternately, in another embodiment, a dry bimodal silicaxerogel is interacted with a chromium compound, such aschromium(III)acetylacetonate, t-butyl chromate or dicumene chromium,that has been dissolved in an organic solvent, such as n-hexane,pyridine, acetone, methanol and the like. Interacting the bimodal silicaxerogel support with a chromium compound solubilized in an organicsolvent is the preferred method of producing the catalyst in order toavoid destroying or adversely affecting the bimodality of the silicaxerogel particle. Chromium(III)acetylacetonate is a preferred chromiumcompound interacted with the bimodal silica xerogel. If the chromiumcompound is a zerovalent chromium compound, the interaction between thezerovalent chromium compound and the bimodal silica xerogel preferablyis performed in a dry, inert atmosphere, such as under nitrogen or undera vacuum, and the resulting catalyst preferably is maintained in a dryinert atmosphere or under vacuum until use.

In general, the preparation of a polymerization catalyst forpolymerizing a 1-olefin, and having a bimodal silica xerogel of thepresent invention as a support material is illustrated as follows:

Chromium(III)acetylacetonate-type catalysts were prepared by firstadding a solution of chromium(III)acetylacetonate (Cr(acac)₃) in anon-polar organic solvent, like hexane or methylene chloride, to aslurry of the bimodal silica xerogel dispersed in a nonpolar organicsolvent. After agitating the resulting mixture for a sufficient time toallow the Cr(acac)₃ to interact with the bimodal silica xerogel, thenon-polar organic solvent was evaporated, and the resultingchromium-based polymerization catalyst was dried at a slightly elevatedtemperature, such as at about 100° C. in a vacuum oven. The driedcatalyst then was heat activated. The high temperature activation of thechromium-based catalyst can be omitted when the chromium-based catalystis used with a metal alkyl cocatalyst.

Several chromium-based polymerization catalysts including a bimodalsilica xerogel support were prepared, with each catalyst containing 1wt. % chromium. These chromium-based catalysts were used in thepolymerization 1-olefins, as illustrated by the polymerization ofethylene.

In particular, the preparation of a chromium-based catalyst including abimodal silica xerogel of the present invention as the catalyst supportis illustrated in the following Example 17.

EXAMPLE 17

Preparation of Chromium-Based Catalyst Including a Bimodal SilicaXerogel Support

The bimodal silica hydrogel of Example 2 was washed with water until thesodium ion concentration of the wash water was less than 10 ppm. Thebimodal silica hydrogel then was rinsed with acetone until the amount ofwater in the acetone rinse was less than 1% by weight water. The bimodalsilica hydrogel was not filtered to dryness prior to rinsing withacetone in order to preserve the bimodal pore structure anddistribution. The resulting bimodal silica xerogel then was dried at atemperature of about 100° C. for about 2 hours in a vacuum oven.

Next, the dried bimodal silica xerogel of Example 2 (5.85 g) was added,under a nitrogen atmosphere, to a flask equipped with a stirrer.Methylene chloride (66 g) was added to the flask, and the resultingmixture was agitated to form a slurry. Then, fifty-one milliliters of asolution of Cr(acac)₃ in methylene chloride was added to the agitatedslurry. The Cr(acac)₃ solution included 1.17 mg (milligram) of chromiumper ml. of solution, and was prepared by dissolving Cr(acac)₃ (1.90 g,including 11.89% by wt. chromium) in 192.4 ml of methylene chloride. Theamount of Cr(acac)₃ solution added to the agitated slurry was sufficientto provide 0.0597 g of chromium, thereby providing a catalyst having achromium content of 1.02% by weight.

The resulting slurry including the Cr(acac)₃ was agitated at ambienttemperature for about 30 minutes. Agitation was stopped, and thechromium based catalyst was allowed to settle from the slurry. Thesupernatant liquid was essentially colorless, thereby demonstrating thatthe chromium was deposited onto the bimodal silica xerogel support.

Agitation then was resumed, the slurry was heated, and, using a nitrogengas purge, the methylene chloride was evaporated. The resultingchromium-based polymerization catalyst was dried at 50° C. to 70° C.,under a nitrogen blanket, for up to about 30 minutes to provide 5.1 g ofa purple, free-flowing powder. The chromium-containing catalyst wasactivated in a glass activator at 820° C. for 6 hours in air by aprocedure well-known to those skilled in the art.

In addition to the above-described chromium-based catalyst of Example 17that included the bimodal silica xerogel of Example 2 as a support, fiveother chromium-based catalysts were prepared in an essentially identicalmanner, and used in the polymerization of ethylene. For example, achromium-based catalyst was prepared using the bimodal silica xerogel ofExample 3 and using the bimodal silica xerogel of Example 8, byinteracting the xerogel with Cr(acac)₃ in the method described above. Achromium-based catalyst also was prepared from thecommercially-available silica xerogel, Cr(acac)₃ on POLYPOR® Silica Gel,a standard silica having a pore radius distribution of from about 70 Åto 220 Å, and available from USI Chemical Division, Quantum ChemicalCorp., New York, N.Y. The POLYPOR® Silica Gel is not a bimodal silicaxerogel.

In addition, chromium-based catalysts were prepared from the bimodalsilica xerogels prepared in Examples 18 and 19. The polymerizationcatalysts including the bimodal silica xerogels of Examples 18 and 19were prepared by interacting the bimodal silica xerogel with Cr(acac)₃in the method described above in Example 17.

COMPARATIVE EXAMPLE 18

A silica hydrogel was prepared by simultaneously adding a 40% by weightaqueous sodium silicate solution and a 4% by weight aqueous sulfuricacid solution to a heel of water in such a manner that an excess ofsodium silicate and an excess of sulfuric acid alternated throughout theaddition of the sodium silicate solution and the sulfuric acid solutionto the reaction mixture. By this method, the pH of the reaction mixturevaried from acidic to alkaline causing at least two distinctprecipitations of silica hydrogel. This method provided no aging stepafter the first precipitation of a silica hydrogel. The resulting silicaxerogel had a broad pore radius distribution that exhibited bimodalcharacteristics. However, as illustrated in the pore radius distributioncurve of FIG. 25, the separation between the major pore radiusdistribution peaks is not distinct, such as greater than about 20 Å. Thesilica xerogel of Example 18 exhibited a P.V. of 2.37 cc/g and a S.A. of639 m² /g.

EXAMPLE 19

A silica xerogel was prepared in an identical manner to the silicaxerogel of Example 2, except the aging period after the secondprecipitation and prior to heat aging in Example 19 was 2 hours ratherthan 24 hours. The resulting silica xerogel exhibited a bimodal poreradius distribution, as illustrated in FIG. 26, and had a P.V. of 1.93cc/g and a S.A. of 264 m² /g.

Ethylene polymerizations were performed using the chromium-basedcatalysts prepared from the silica xerogels of Examples 2, 3, 8 and 19and of Comparative Example 18. The resulting polyethylene resins werecompared to a polyethylene resin produced from a chromium-based catalystusing a standard silica gel support (POLYPOR® Silica Gel) to determinewhether a resin prepared from a catalyst supported on a bimodal silicaxerogel demonstrated a broad or bimodal molecular weight distribution.The results of the ethylene polymerizations are illustrated in TABLE 3.

In general, the chromium-based catalysts including a bimodal silicaxerogel support were used in the polymerization of ethylene in aconventional particle form (slurry) process. The slurry process iswell-known to a person skilled in the art, and is described in severalprior patents, such as U.S. Pat. No. 3,644,323. In particular, a slurryprocess polymerization is performed in a liquid organic medium at atemperature from about 65° C. to about 110° C. (about 150° F. to about230° F.). The chromium-based catalyst is suspended in the organicmedium, and the polymerization reaction is conducted at a pressuresufficient to maintain the organic diluent and at least a portion of theolefin in the liquid phase. The mole percent ethylene in the reactor isgenerally maintained at from about 2 to about 25. Hydrogen can be addedto the polymerization reaction. The molar ratio of hydrogen to ethylenein the reactor is generally maintained between 0.25 and 1.0. While notnecessary for polymerization, a reducing agent is generally included inthe reactor with the catalyst.

The organic medium used in the polymerization usually is an alkane orcycloalkane, such as, but not limited to, propane, butane, isobutane,pentane, isopentane, cyclohexane, methylcyclohexane, and the like; andcombinations thereof. The organic medium is chosen such that, under theparticular polymerization conditions employed, the polymer is insolublein the organic medium and is readily recoverable in the form of solidparticles. Isobutane is a particularly advantageous organic medium.Pressures in the reactor typically range from about 100 to about 800psig, and catalyst concentrations can range from about 0.0001 to about 1percent, based on the total weight of the reactor contents. Thepolymerizations can be conducted as a batch, a continuous or asemi-continuous operation.

                                      TABLE 3                                     __________________________________________________________________________    ETHYLENE POLYMERIZATION WITH CHROMIUM                                         CATALYST ON A BIMODAL SILICA XEROGEL SUPPORT                                            Polymeriza-                                                         Catalyst  tion Time                                                           Support                                                                              P.V.                                                                             (minutes).sup.2)                                                                    Productivity.sup.3)                                                                  Reactivity.sup.4)                                                                   MI.sup.5)                                                                         MIR.sup.6)                                                                        Density.sup.7)                                                                     Mw/Mn.sup.8)                        __________________________________________________________________________    Example 2                                                                            2.15                                                                             36    1412   2353  .25 110 --   8.61                                Example 3                                                                            2.65                                                                             48    3050   3813  .34 94  .9620                                                                              10.24                               Example 8                                                                            2.37                                                                             --    --     731   .07 145.7                                                                             .9578                                                                              10.61                               Example 18                                                                           2.37                                                                             51    2606   3067  .32 80.6                                                                              --   8.72                                Example 19                                                                           1.93                                                                             49    2564   3140  .095                                                                              166 .9619                                                                              11.3                                POLYPOR ®                                                                        2.40                                                                             60    2192   2192  .76 73  .9624                                                                              7.84                                Silica Gel                                                                    (Standard)                                                                    __________________________________________________________________________     .sup.1) Each catalyst support is interacted with Cr(acac).sub.3 to provid     a catalyst containing 1% Cr; the catalyst was activated at 820° C.     for 6 hours in air;                                                           .sup.2) Polymerization conditions:                                            Temperature: 105 ± 1° C. (221° F.)                           Ethylene: about 16.6 mole percent                                             Total Pressure: 550 psi                                                       Catalyst Charge: 0.0211 to 0.078, Isobutane diluent, except for Example 8     Example 8 polymerizaed at 5 mole percent ethylene at 100° C.           (211° F.);                                                             .sup.3) Productivity is expressed as g (gram) polyethylene per g catalyst     .sup.4) Reactivity expressed as g polyethylene per g catalyst per hour;       .sup.5) Melt index;                                                           .sup.6) Melt index ratio (HLMI/MI);                                           .sup.7) Density is expressed as g per cubic centimeter; and                   .sup.8) Ratio of weight average molecular weight to number average            molecular weight.                                                        

In accordance with the present invention, polyethylene resins havinggood processing characteristics, high density and high resistance toenvironmental stress cracking were obtained from a chromium-basedcatalyst including a bimodal silica xerogel support. Whereaschromium-based catalysts are known, surprisingly and unexpectedly, ithas been found that by using a bimodal silica xerogel of the presentinvention as the catalyst support, the resulting chromium-basedcatalysts produced resins that exhibit a broad or bimodal molecularweight distribution, thereby having excellent processingcharacteristics, and also have high densities and high resistance toenvironmental stress cracking. The combination of desirable processingcharacteristics, plus both high density and high resistance to stresscracking, makes the resins useful in blow molding applications, andparticularly useful in the production of blown bottles.

From the data tabulated in TABLE 3, a chromium-based catalyst having abimodal silica xerogel of the present invention as a support producedpolyethylene resins having densities of at least 0.9578, excellent Mw/Mnratios, and acceptable flow properties and shear response. Moreparticularly, polyethylene resins produced in accordance with thepresent invention have exhibited densities from 0.9578 to 0.9620, andMw/Mn ratios from greater than 8.5 to greater 11.25. A polymer having ahigh Mw/Mn ratio indicates that the polymer has good flow properties,i.e. processability. For comparative purposes, the polyethylenemanufactured using a catalyst having a POLYPOR® Silica Gel supportexhibits a Mw/Mn ratio of 7.84. Such a Mw/Mn ratio is recognized bythose skilled in the art as being exhibited by a polyethylene havingexcellent flow properties. Surprisingly, from TABLE 3, it is observedthat a polyethylene manufactured from a catalyst having a bimodal silicasupport of the present invention exhibits an improved Mw/Mn ratio offrom 8.61 to 11.3, and therefore exhibits improved processability andflow properties over a polyethylene prepared from a catalyst having aPOLYPOR® Silica Gel support.

In addition to having a density and a Mw/Mn ratio in the above ranges,the polyethylene resins also exhibited melt indexes (pellet) (MI) in therange of 0.07 to 0.34 and melt index ratios (MIR) in the range 80.6 to166. The MIR generally is considered an approximation of molecularweight distribution, and is the ratio between the high load melt index(HLMI) determined in accordance with ASTM D1238-57T, condition F, andthe MI determined in accordance with ASTM D1238-57T, condition E. Ingeneral, with resins having a comparable MI, a polymer with a broadermolecular weight distribution has a higher MIR and better flowproperties than a lower MIR counterpart. For example, a catalystprepared from the bimodal silica xerogel of Example 8 provided apolyethylene resin having a polyethylene resin having a MI of 0.07 and aHLMI of 10.2, to provide a MIR of 145.7, thus indicating a broad orbimodal molecular weight distribution.

In general, from the data illustrated in TABLE 3, it has been shown thata chromium-based catalyst, including a bimodal silica xerogel of thepresent invention as a support, exhibits a greater reactivity andprovides broad or bimodal polyolefin resins.

Utilizing the above-described catalysts, it is possible to produceeasily processible and highly useful polyethylene resins having a highdensity, a high Mw/Mn ratio, and a high resistance to environmentalstress cracking. For example, polyethylene resins having densities inthe range 0.958 to 0.962, Mw/Mn ratios from 8.61 to 11.3 with meltindexes from 0.07 to 0.34 and melt index ratios from 80 to 166 areobtained thereby. It is especially desirable that these polyethyleneresin products demonstrate a broad or bimodal molecular weightdistribution, and can be obtained using a chromium-based catalystincluding a bimodal silica xerogel support. It is extremely useful andadvantageous from a commercial standpoint that the bimodal silicaxerogel support of the catalyst provides these highly desirable resultsbecause necessity of blending polyethylene resins having differentaverage molecular weights thereby can be eliminated. Accordingly, apolyethylene resin having a broad or bimodal molecular weightdistribution can be produced in a single polymerization reaction byutilizing a bimodal silica xerogel of the present invention as thecatalyst support, wherein the bimodal silica xerogel particle has twoaverage pore radius distributions differing by at least 20 Å.

Obviously, many modifications and variations of the invention ashereinbefore set forth can be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated by the appended claims.

We claim:
 1. A method of preparing a silica gel particle, comprising thesteps of:(a) forming a silica hydrogel by neutralizing an aqueoussolution of a silicate, wherein the silicate includes a cation selectedfrom the group consisting of alkali metals, ammonium, and combinationsthereof, by adding the silicate solution to a first aqueous acidsolution to raise the pH of the first aqueous acid solution until thesilica hydrogel is precipitated; (b) aging the silica hydrogel of step(a) in the resulting solution of step (a) for a time sufficient toprovide an aged silica hydrogel having a first average pore radius; (c)adding silicate solution as defined in step (a) to the aged hydrogel andsolution of step (b) to raise the pH thereof to at least about 9; (d)neutralizing the resulting hydrogel and solution of step (c) by adding asecond aqueous acid solution thereto to sufficiently lower the pHthereof to further precipitate the silicate as a hydrogel having asecond average pore radius on the hydrogel of step (b); and, (e)heat-aging the resulting hydrogel and solution of step (d) for asufficient time and at a sufficiently high temperature to fix therespective first and second average pore radii of the hydrogel of step(d), wherein the first average pore radius is different from the secondaverage pore radius.
 2. The method of claim 1 wherein the silicate isselected from the group consisting of sodium silicate, potassiumsilicate, lithium silicate, ammonium silicate, and combinations thereof.3. The method of claim 1 wherein the aqueous silicate solution of step(a) comprises from about 10% to about 40% by weight of silica, as SiO₂.4. The method of claim 1 wherein the first aqueous acid solutioncomprises an acid selected from the group consisting of sulfuric acid,nitric acid, phosphoric acid, hydrochloric acid, and combinationsthereof.
 5. The method of claim 1 wherein the first aqueous acidsolution includes from about 2% to about 12% by weight acid.
 6. Themethod of claim 1 wherein a sufficient amount of the silicate solutionis added to the first aqueous acid solution in step (a) to raise the pHof the first aqueous acid solution to about 4.5 to about 7 after theaddition of the silicate solution.
 7. The method of claim 1 wherein thesilicate solution is added to the first aqueous acid solution in step(a) over a time period of from about 5 minutes to about 20 minutes. 8.The method of claim 1 wherein the silicate comprises sodium silicate;the first aqueous acid solution comprises from about 2% to about 12% byweight sulfuric acid; and the pH of the first aqueous acid solution instep (a) is raised to about 6 to about 7 after the addition of thesilicate solution.
 9. The method of claim 8 wherein the sodium silicatehas a ratio of Na₂ O to SiO₂ in the range of about 1:3 to about 1:3.75.10. The method of claim 1 wherein the silica hydrogel of step (a) isaged in step (b) for about 15 minutes to about 4 hours.
 11. The methodof claim 1 wherein the silica hydrogel of step (a) is aged in step (b)to provide an aged silica hydrogel having a first pore radiusdistribution in the range of about 20 Å to about 150 Å.
 12. The methodof claim 11 wherein the silica hydrogel of step (a) is aged in step (b)to provide an aged silica hydrogel having a first average pore radius ofabout 40 Å to about 125 Å.
 13. The method of claim 1 wherein sufficientsilicate solution is added in step (c) to raise the pH of the agedsilica hydrogel and solution of step (b) to about 10 to about
 11. 14.The method of claim 1 wherein the silicate solution in step (c) is addedover a time period of from about 10 minutes to about 2 hours.
 15. Themethod of claim 1 wherein the second aqueous acid solution of step (d)comprises about 2% to about 12% by weight of an acid selected from thegroup consisting of sulfuric acid, nitric acid, phosphoric acid,hydrochloric acid, and combinations thereof.
 16. The method of claim 1wherein a sufficient amount of the second aqueous acid solution is addedin step (d) to lower the pH of the resulting hydrogel and solution ofstep (c) to about 5 to about
 7. 17. The method of claim 1 wherein theheat aging of step (e) is conducted at a temperature in the range ofabout 70° C. up to about 100° C.
 18. The method of claim 1 wherein theheat aging of step (e) is conducted for a time of about 30 minutes toabout 4 hours.
 19. The method of claim 1 wherein the second average poreradius of the silica gel particle is greater than the first average poreradius of the silica gel particle.
 20. The method of claim 1 wherein thesilica hydrogel of step (d) has a first pore radius distribution and asecond pore radius distribution wherein the second pore radiusdistribution is in the range of about 50 Å to about 500 Å.
 21. Themethod of claim 1 wherein the silica gel particle has a bimodal poreradius distribution wherein the difference between the second averagepore radius and the first average pore radius is at least about 20 Å.22. The method of claim 1 further comprising the step of aging thehydrogel of step (c) in the resulting solution of step (c) for asufficient time to allow the resulting solution of step (c) to interactwith the hydrogel of step (c).
 23. The method of claim 1 furthercomprising the step of aging the hydrogel of step (d) in the resultingsolution of step (d) for a time sufficient to provide an aged silicahydrogel having a first average pore radius and a second average poreradius prior to heat-aging the hydrogel in step (e).
 24. The method ofclaim 23 wherein the hydrogel of step (d) is aged for up to about 24hours prior to heat-aging the hydrogel in step (e).
 25. The method ofclaim 1 wherein the aqueous silicate solution further comprises atitanium-containing compound, a zirconium-containing compound, avanadium-containing compound, or a combination thereof, in an amountsuch that the sum of the titanium, zirconium and vanadium concentrationsin the aqueous silicate solution is about 5 wt. % or less, based on theweight of SiO₂ in the silicate solution.
 26. The method of claim 25wherein the titanium-containing compound, the zirconium-containingcompound or the vanadium containing compound is selected from the groupconsisting of titanium sulfate, an alkali metal titanium oxalate,ammonium titanium oxalate, ammonium tetraoxalatozirconate(IV)tetrahydrate, an alkali metal tetraoxalatozirconate(IV), a vanadiumoxytrihalide, a vanadium pentahalide, a vanadium trihalide, andcombinations thereof.
 27. A method of preparing a silica xerogelparticle having a bimodal pore radius distribution, comprising the stepsof:(a) forming a silica hydrogel by neutralizing an aqueous solution ofa silicate, wherein the silicate includes a cation selected from thegroup consisting of alkali metals, ammonium, and combinations thereof,by adding the silicate solution to a first aqueous acid solution toraise the pH of the first aqueous acid solution until the silicahydrogel is precipitated; (b) aging the silica hydrogel of step (a) inthe resulting solution of step (a) for a time sufficient to provide anaged silica hydrogel having a first average pore radius; (c) addingsilicate solution as defined in step (a) to the aged hydrogel andsolution of step (b) to raise the pH thereof to at least about 9; (d)neutralizing the resulting hydrogel and solution of step (c) by adding asecond aqueous acid solution thereto to sufficiently lower the pHthereof to further precipitate the silicate as a hydrogel having asecond average pore radius on the hydrogel of step (b); (e) heat-agingthe resulting hydrogel and solution of step (d) for a sufficient timeand at a sufficiently high temperature to fix the respective first andsecond average pore radii of the hydrogel of step (d), wherein the firstaverage pore radius is different from the second average pore radius;(f) washing the heat-aged hydrogel of step (e) with a sufficient amountof water until the cation concentration in the wash water is less thanabout 10 parts per million; (g) displacing the wash water from theheat-aged hydrogel of step (e) by a method selected to avoid substantialcollapse of the pores of the heat-aged hydrogel and to reduce the amountof water in the heat-aged hydrogel to less than 1% by weight water; and(h) drying the product of step (g) to provide the silica xerogel. 28.The method of claim 27 wherein the silica hydrogel of step (a) is agedin step (b) to provide an aged silica hydrogel having a first poreradius distribution in the range of about 20 Å to about 150 Å.
 29. Themethod of claim 28 wherein the silica hydrogel of step (a) is aged instep (b) to provide an aged silica hydrogel having a first average poreradius of about 40 Å to about 125 Å.
 30. The method of claim 27 whereinthe second average pore radius of the silica gel particle is greaterthan the first average pore radius of the silica gel particle.
 31. Themethod of claim 27 wherein the silica hydrogel of step (d) has a firstpore radius distribution and a second pore radius distribution, whereinthe second pore radius distribution is in the range of about 50 Å toabout 500 Å.
 32. The method of claim 27 wherein the silica gel particlehas a bimodal pore radius distribution wherein the difference betweenthe second average pore radius and the first average pore radius is atleast about 20 Å.
 33. The method of claim 27 wherein the wash water isdisplaced in step (g) by washing the heat-aged hydrogel with awater-miscible organic liquid capable of replacing water in the pores ofthe heat-aged hydrogel.
 34. The method of claim 33 wherein thewater-miscible organic liquid is selected from the group consisting ofacetone, methanol, ethanol, isopropyl alcohol, and combinations thereof.35. The method of claim 33 wherein the water-miscible organic liquiddisplaces about 90% to about 95% of the water from the heat-agedhydrogel, then the resulting silica gel is treated with a ketal of theformula RC(OR)₂ R, wherein R is the same or different and is an alkylgroup including one to five carbon atoms, in the presence of a trace ofacid.
 36. The method of claim 35 wherein the ketal is selected from thegroup consisting of 2,2-dimethoxypropane, 2,2.dimethoxybutane,2,2-diethoxypropane, 2-methoxy-2-ethoxypropane, and combinationsthereof.
 37. The method of claim 27 wherein the wash water is displacedin step (g) by azeotropic distillation with a compound capable offorming an azeotrope with water.
 38. The method of claim 37 wherein thecompound capable of forming an azeotrope with water is ethyl acetate orbenzene.
 39. The method of claim 27 wherein the wash water is displacedin step (g) by contacting the heat-aged hydrogel with a ketal of theformula (RC(OR)₂ R, wherein R is the same or different and is an alkylgroup including one to five carbon atoms, in the presence of a trace ofacid.
 40. The method of claim 27 wherein step (h) is performed at atemperature of about 80° C. to about 110° C.
 41. A method of preparing acatalyst for the polymerization or copolymerization of one or more1-olefins comprising the steps of:(a) forming a silica hydrogel byneutralizing an aqueous solution of a silicate, wherein the silicateincludes a cation selected from the group consisting of alkali metals,ammonium, and combinations thereof, by adding the silicate solution to afirst aqueous acid solution to raise the pH of the first aqueous acidsolution until the silica hydrogel is precipitated; (b) aging the silicahydrogel of step (a) in the resulting solution of step (a) for a timesufficient to provide an aged silica hydrogel having a first averagepore radius; (c) adding silicate solution as defined in step (a) to theaged hydrogel and solution of step (b) to raise the pH thereof to atleast about 9; (d) neutralizing the resulting hydrogel and solution ofstep (c) by adding a second aqueous acid solution thereto tosufficiently lower the pH thereof to further precipitate the silicate asa hydrogel having a second average pore radius on the hydrogel of step(b); (e) heat-aging the resulting hydrogel and solution of step (d) fora sufficient time and at a sufficiently high temperature to fix therespective first and second average pore radii of the hydrogel of step(d), wherein the first average pore radius is different from the secondaverage pore radius; (f) washing the heat-aged hydrogel of step (e) witha sufficient amount of water until the cation concentration in the washwater is less than about 10 parts per million; (g) displacing the washwater from the heat-aged hydrogel of step (e) by a method selected toavoid substantial collapse of the pores of the heat-aged hydrogel and toreduce the amount of water in the heat-aged hydrogel to less than 1% byweight water; (h) drying the product of step (g) to provide a silicaxerogel; and (i) depositing a chromium-containing compound on the silicaxerogel of step (h) to provide the polymerization catalyst.
 42. Themethod of claim 41 wherein the polymerization catalyst of step (i) iscalcinated at about 400° C. to about 1000° C. for up to about 12 hours.43. The method of claim 41 wherein the chromium containing compound is azerovalent chromium compound, a chromium(II) compound, a chromium(III)compound, a chromium(VI) compound, or a combination thereof.
 44. Themethod of claim 41 wherein the chromium-containing compound is selectedfrom the group consisting of chromocene, cycopentadienyl chromiumethoxide, chromium(III)acetate, chromium(III)propionate,chromium(III)acetylacetonate, chromium(IV)trioxide, ammonium dichromate,bis-triphenylsilyl chromate, dibenzene chromium, ditoluene chromium,di-o-xylene chromium, di-p-xylene chromium, dicumene chromium,dimesitylene chromium, di(n-propylbenzene)chromium,di(1,3,5-triethylenebenzene) chromium, di(1,3-diethyl-4-hexylbenzene)chromium, di(1,3-dipentylbenzene) chromium, di-(1,3,5-trihexylbenzene)chromium, di(hexamethylbenzene) chromium, and mixtures thereof.
 45. Themethod of claim 41 wherein the aqueous solution of the silicate furthercomprises a titanium-containing compound, a zirconium-containingcompound, a vanadium-containing compound, or a combination thereof, inan amount such that the sum of the titanium, zirconium and vanadiumconcentrations in the aqueous silicate solution is about 5 wt. % orless, based on the weight of SiO₂ in the silicate solution.